An SMA (also referred to as a smart metal, memory metal, memory alloy, muscle wire, and smart alloy) is an alloy that “remembers” its original shape when deformed, and returns to its original shape when heated. The two main types of shape-memory alloys are copper-aluminium-nickel, and nickel-titanium (NiTi) alloys, but SMAs can also be created by alloying zinc, copper, gold and iron. Under heating, an element comprising SMA in the martensite state will start to deform at a first threshold temperature (As) and change to the austenite state by the time it reaches a second (higher) temperature (Af). The SMA element in the austenite state may be contracted so that it is shorter in the austenite state than when in the martensite state. Sometimes, the SMA element in the austenite state may be twisted. The SMA element will remain in the austenite state until the SMA element is cooled below a certain threshold temperature (Ms), which is less than Af, when it will start to return to the martensite state, and will return back to the martensite state by the time the SMA element reaches another particular temperature (Mf), which is generally less than As.
For example, for a NiTi SMA element, As can be approximately 50° C., Af can be approximately 70° C., Ms can be approximately 60° C., and Mf can be approximately 40° C. However, the skilled person will understand that these transition temperatures can vary widely, based both on the type of alloy used in the SMA element as well as the processing of the alloy. For example, the transition temperatures for a NiTi SMA element can vary by as much as 40° C. based on processing.
Active material elements such as SMA elements are used in various devices, such as actuators and smart devices, to cause displacement of structural members of the devices by activation of the active elements. This may be done, for example, by passing current through an SMA element, thereby causing it to heat (which may be referred to as “Joule heating”), resulting in deformation of the SMA element and the application of force to the members it is attached to.
Uneven cooling or an insufficient return or biasing force may cause SMA elements to buckle and bow out of radial alignment. For example, buckling may occur in a device having multiple SMA elements in parallel if the ambient temperature around the different SMA elements varies, as this can cause some of the SMA elements to cool faster and expand while the other SMA elements remain contracted. If, as a result of buckling, SMA elements cross or touch a conductive body, this can cause a short circuit when current is passed through the SMA element.
This is illustrated in FIG. 1. FIGS. 1a, 1b, 1c and 1d are cross-sectional views of two SMA wires 100a and 100b connected to support element 110 at one end and to conductive surface 120 at the other end. FIG. 1a shows SMA wires 100a and 100b before activation. FIG. 1b shows SMA wires 100a and 100b after activation. FIG. 1c shows SMA wires 100a and 100b after activation and where wire 100a has recovered, but wire 100b has not recovered. In this case, SMA wire 100a has buckled and is touching wire 100b. FIG. 1d shows SMA wires 100a and 100b after activation and where wire 100a has recovered, but wire 100b has not recovered. In this case, SMA wire 100a has buckled and is touching conductive surface 120.
Repeated deformation of SMA elements can cause them to break, resulting in live, unsupported, SMA strands. This is particularly problematic in the case of multi-wire actuators since, even if one SMA element breaks, the presence of other SMA elements in parallel allows the actuator to continue to function, causing the live ends of the broken SMA element to be subjected to external forces and displacements. The resulting movement may increase the probability of the live ends of the broken SMA element arcing against other surfaces.
Power demands on SMA-based actuators, and in particular on multi-wire actuators, have been increasing, resulting in an increase in applied voltage and current. At high voltage and current levels, personal protection is especially important to prevent injuries, such as ventricular fibrillation, caused by contact with live parts.
Naturally occurring oxidation of the surface of SMA elements may provide a light insulative coating, however, this coating does not provide adequate protection against contact with live SMA strands, particularly for SMA strands at high voltages. In addition, the oxidative coating can often be easily removed by polishing.
While elastomer-based coatings and sheaths for SMA elements, such as those disclosed in U.S. Pat. No. 7,086,885, have been used to increase heat dissipation, lower friction, and increase biocompatibility, there are a number of problems with the use of elastomers to provide electrical insulation.
Elastomer-based coatings and sheaths are generally extruded around a wire. A thick coating may exert spring-like opposing forces that resist deformation of a coated SMA element. A thin coating tends to be relatively weak, and relies on the coated SMA element for support. Failure of the SMA element may lead to failure of the coating or sheath.
Certain elastomers, such as thermoplastic polyurethane, have low glass transition temperatures. If an elastomer is used to coat an SMA element with an activation temperature above the glass transition temperature of the elastomer, a coating or sheath made from that elastomer will tend not to withstand activation of the SMA element. This is particularly problematic for SMA elements with high activation temperatures, which are used in high temperature environments. For example, a starter motor of a car should be able to function in temperatures ranging from about −40° C. to about 125° C. Accordingly, an SMA element used in this environment should have an activation temperature above 125° C. to ensure that the SMA element only deforms when desired, and not simply due to the high temperature of its surroundings. Any coating or sheath used for such an SMA element would accordingly need to have a high glass transition temperature.
In addition, some elastomer coatings, such as ethylene propylene diene terpolymer (EPDM), restrict conductive heat transfer in fluid environments. This limits the effectiveness of any liquid heat sink used to reduce SMA element recovery time if the SMA element is coated with such an elastomer.
Furthermore, factory-applied coatings or sheaths may complicate SMA element installation and assembly, making it more difficult to route SMA elements through low clearance holes or achieve low-resistance electrical hook-up connections.